An In - vivo study to evaluate the force decay of three different orthodontic elastomeric ligatures.

Abstract: Background: In vitro studies conducted under simulated oral environment under estimated the force decay of elastomeric ligatures. There are only few invivo studies that are reported evaluating the force decay of elastomeric ligatures.  Aim & Objectives:  The aim of this in-vivo study was to evaluate the force decay of three different clear elastomeric ligature modules stretched over the lower incisor brackets intra orally over three different time periods at 24 hrs, 7 days and 21 days by comparing it with the force levels evaluated at as received conditions . Materials and methods: Ten patients undergoing fixed orthodontic treatment with  Roth Prescription, 0.022" slot preadjusted edgewise stainless steel brackets   with aligning and levelling completed and   0.019" x 0.025" SS arch wire placed passively in the lower arch were included in the study. The three types of elastomeric ligature modules formed the three study groups [Group I: Molded“O” (Ormco, USA),Group II: Alastik (3M Unitek, USA)&Group III: Uni-stick (American orthodontics, USA)] Each group was assigned with a specific lower incisor in each patient and the respective modules were used in the teeth during the study period. The force levels required to stretch the modules to a predetermined length of 4.1mm were evaluated at 4 different time intervals at 0 hour (as received), 24, 7 days, and 21 days after intra-oral use using a universal testing machine (Instron model 1112, Instron Corp.). The mean force values and standard deviations for all the groups at each time period.  The mean percentage of force decay between time periods for each study group was calculated. One way ANOVA followed by Post hoc Tukey HSD was performed for within the group and between group comparisons. Paired T test was done for pairwise comparisons. (P value≤ 0.05) Results: The mean force level recorded were high at the base line level with as received modules before inserting into the patients mouth for all the three study groups.  The mean force level was 766.50 ± 3.659 gms, 650.50±2.759 gms and 594 .10 ± 4.630 gms for group I, II & III respectively in the as received condition. The percentage of force decay was 32.94 % at 24 hrs and 69.85% at 7 days and 79.31% at 21 days for Group I. A force decay of 31.36%, 70.76% and 79.83% from the base line value was noted in group II at the same time intervals. In Group III The force decay rate at 24 hrs, 7 days and 21 days were 33.29%, 66.10% and 77.12% respectively. One way ANOVA for within group and between group comparison showed a significant difference between  the force levels recorded at different time intervals within and between all the three study groups. Post Hoc Tukey HSD revealed a significant difference between the three groups at all the time intervals tested except at 7 days and 21 days where the force levels exerted by the group II and Group III samples were not significantly different. Paired sample T test for pairwise comparison of force exerted between different time intervals in all the three groups revealed a significant difference. Conclusion: Group I clear modules exhibited significantly high mean force and less force decay compared to other groups at all four time intervals tested. The elastomeric modules exhibited significant force decay after 24 hours, 7days and 21 days of intra-oral use except at 7 days and 21 days where the force levels exerted by the group II and Group III samples were not significantly different. The  forces at the end of three weeks of intra oral use appeared to be insufficient for effective torquing effect and severe rotational corrections in all the three study groups and hence changing modules sfter three weeks may be beneficial in those cases